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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS66 KMFR 210111
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
611 PM PDT Tue Apr 20 2021

.UPDATE...Updated the aviation section.

&&

.AVIATION...For the 21/00Z TAF Cycle...VFR ceilings and a few
showers will linger in northern California this evening. Also,
LIFR/IFR ceilings and visibilities cover most of the coast north of
Cape Blanco. This coastal stratus is expected to expand along the
coast and in to the Coquille Valley overnight. It will then burn off
back to the beaches during the morning with lingering IFR again
expected Wednesday afternoon north of Cape Blanco.

Elsewhere, VFR conditions are expected through Wednesday evening.
-DW

&&

.MARINE...Updated 200 PM PDT Tuesday 20 Apr 2021...A thermal
trough near the coast will support strong north winds through
Wednesday, especially south of Cape Blanco. Winds and seas will
continue to increase into the evening, with hazardous seas
developing beyond 5 NM from shore and south of Port Orford. Very
steep and hazardous seas are expected again Wednesday afternoon.
Winds will ease some Thursday into Friday, but steep seas will
linger through Thursday evening. Winds will become southerly
Saturday, when the first in a series of fronts will move onshore.
-Sven

&&

.PREV DISCUSSION... /Issued 218 PM PDT Tue Apr 20 2021/

DISCUSSION...The latest radar image shows isolated a line of
showers (likely from a deformation zone) aligned from west to east
in southern Jackson and Klamath county. Additionally, showers are
also confined to northern California. Over time, the line of showers
in the above mentioned areas will continue to move south into
California with decreasing cloud cover. Meanwhile mainly isolated
showers will continue through this afternoon in northern California.

This is all due to a weak upper low located along the
California/Oregon border, what is bringing a weak trigger and cooler
air aloft. The combination of the two will bring enough instability
to generate additional isolated showers and a isolated storm or two
this afternoon.

Any leftover isolated showers and storms will end early this evening
with dry weather expected tonight with clearing skies for most of
the area. The exception will be along the coast from about Cape
Blanco north where marine stratus is expected to return. The marine
stratus could push inland into the Coquille Basin, but that should
be the extent of it. Elsewhere it will be clear.

After today, were back to dry and warm weather until the end of the
week. A weak disturbance within a northwest flow will move through
late Wednesday night with the main effect being an increased onshore
flow. Therefore, we can expect marine stratus to make it`s way
further inland to include most, if not the entire Umpqua Basin by
Thursday morning.

Friday will be dry as a ridge axis moves overhead. This will quickly
move east Friday evening as an upper trough approaches from the
southwest. The evidence is pretty solid we`ll get our first
measurable rainfall in our forecast area this morning, starting at
the coast later Friday evening, then pushing inland mainly west of
the Cascades Friday night. -Petrucelli

Extended discussion...Sat 24 Apr through Tue 27 Apr 2021.

The overall pattern will change to a much wetter regime in the
extended period. The offshore long wave ridge which brought the dry
weather to the area will break down and move onshore Friday.  This
will open the storm door and the upstream upper trough will move
onshore Sunday night. Short waves ejecting from the trough will push
a series of fronts onshore beginning Friday night. Sunday looks to
be the wettest day overall.

The trough will move out to the east Monday and a weak ridge will
build into the west coast. This looks to be a dirty ridge; short
waves moving over the top of it will continue to support frontal
activity that will bring more precipitation to the area. A
significant front will move onshore Tuesday.

The ridge will break to the east of the area Tuesday, but it will
build over the intermountain states as the upstream trough deepens
offshore. Eventually there will be a break in the action, but the
deterministic models and their ensembles indicate that this won`t
happen until next Thursday.

The 8 to 14 day outlook, valid Wed 28 Apr to Tue 04 May, shows
greater chances for above normal temperatures and above normal
precipitation for southern Oregon, and greater chances for above
normal temperatures and near normal precipitation for northern
California.

FIRE WEATHER...Updated 300 PM PDT Tue 20 Apr 2021...As evidenced by
the 1400+ acre wildfire (Ponina) that ignited near Beatty/Bly
yesterday, early spring dryness has led to abnormally dry fuel
conditions area wide, but especially across the east side. Contacts
with local fuels officials today indicated moderate fire danger over
portions of the west side and in most of NorCal, while FWZs 624/625
below 5000 feet in Oregon and FWZ 285 in California all indicated
high fire danger.

High pressure will build in tonight, initiating a warming and drying
trend that will continue into Friday evening. It will also be breezy
in the afternoons and evenings, especially Thursday. This could
bring borderline Red Flag conditions to portions of the east side.

A cooler and much wetter pattern will initiate Friday night as the
first of several systems moves onshore. At this time it looks like
this pattern will persist through at least the middle of next week.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...
CA...None.

Pacific Coastal Waters...Small Craft Advisory until 11 AM PDT
     Thursday for PZZ350-356-370-376.
     Hazardous Seas Warning until 11 AM PDT Thursday for PZZ356-376.

$$

MAP/DW/BMS